1. Field of Invention
The present invention relates to an optical pickup device having among others a light source unit having an arrangement in which a semiconductor laser and a light-receiving element are incorporated in a common package.
2. Related Art
As an optical pickup device used in the recording and reproduction of an optical disk such as a CD, a DVD, and an MO, an optical pickup device is known which has a light source unit having an arrangement in which optical elements such as a semiconductor laser, a light-receiving element, and a hologram element are incorporated in a package. Such an optical pickup device is disclosed in, for instance, Unexamined Japanese Patent Unexamined Publication Hei.3-278330.
In the optical pickup device disclosed Unexamined Japanese Patent Unexamined Publication Hei.3-278330, a hologram element and an objective lens are arranged in that order from the semiconductor laser toward the optical disk. In addition, an objective lens, a hologram element, and a reflecting mirror are arranged in that order from the optical disk toward a light-receiving element (for signal reproduction).
In this optical pickup device, the semiconductor laser is disposed on a semiconductor substrate which is fixed to a heat sink, and these members are integrated as a light source unit. In this light source unit, a sub-heat sink is mounted on the semiconductor substrate, and the semiconductor laser is mounted on its upper surface and emits laser light in a direction parallel to the substrate surface of the semiconductor substrate. The light-receiving element (for signal reproduction) is formed in the rear of the semiconductor laser on the substrate surface of the semiconductor substrate, and the reflecting mirror is disposed above this light-receiving element. Further, in addition to the light-receiving element for signal reproduction, a monitoring light-receiving element for feedback-controlling a laser light output of the semiconductor laser is also formed in the rear of the semiconductor laser.
In this optical pickup device, the laser light (forward laser light) which is made emergent from a front emergent end face of the semiconductor laser is transmitted through the hologram element and is then focused onto the optical disk through the objective lens. The return light from the optical disk is diffracted by the hologram element, and is then made to fall down by the reflecting mirror and is guided to the light-receiving element for signal reproduction. The signal reproduction, tracking error detection, and focusing error detection are effected in correspondence with the result of detection by this light-receiving element.
In addition, in this type of optical pickup device, part of the laser light (backward laser light) which is made emergent from a back emergent end face of the semiconductor laser directly irradiates the monitoring light-receiving element which is formed on the semiconductor substrate surface. On the basis of the result of detection by this monitoring light-receiving element, feedback control is effected so that the laser light output of the semiconductor laser becomes fixed.
The major causes of deterioration of the accuracy of the return light from the optical disk are as follows.
First, the forward laser light and the backward laser light which are made emergent from the front and back emergent end faces of the semiconductor laser have spreads in which the light spreads beyond an effective angle of divergence. In particular, the spread in a direction perpendicular to an active layer is large. In a conventional optical pickup device, of the backward laser light, a light component (an effective bundle of rays) having a large light intensity and having an effective angle of divergence is utilized as monitoring light.
The light component which spreads outside the effective bundle of rays and which is not utilized as the monitoring light is radiated in a direction which deviates from the light-receiving surface of the monitoring light-receiving element. Hence, there is a possibility that part of it is made directly incident upon the light-receiving element for signal reproduction which is disposed in the rear of the semiconductor laser.
If unwanted light (stray light) other than the return light from the optical disk is incident upon the light-receiving element for signal reproduction, the proportion of the noise component included in the output signal from the light-receiving element for signal reproduction increases. Consequently, the accuracy of the return light from the optical disk which is made incident upon the light-receiving element for signal reproduction deteriorates. Specifically, drawbacks occur such as a substantial decline in the S/N ratio as well as instability in the tracking and focus controlling operation of the objective lens.
It is conceivable to dispose the light-receiving element for signal reproduction on the side of the semiconductor laser so that the component of the backward laser light which is not utilized as the monitoring light will not be directly incident upon the light-receiving element for signal reproduction. However, in this case as well, there is a possibility that the light is reflected on an inner wall surface of the package and is made indirectly incident upon the light-receiving element for signal reproduction. Similarly, the forward laser light also as a possibility that part of the light component which spreads outside the effective bundle of rays is made incident upon the light-receiving element for signal reproduction.
Next, in the arrangement in which the return light from the optical disk is guided to the light-receiving element for signal reproduction by using the light-guiding element (the reflecting mirror or the prism), if the accuracy of the mounting position of the light-guiding element or the light-receiving element for signal reproduction is poor, the return light fails to focus on a desired position on the light-receiving element for signal reproduction.
If the focusing position of the return light in the light-receiving element for signal reproduction is not appropriate, the accuracy of the return light from the optical disk which is incident upon the light-receiving element for signal reproduction deteriorates, and it is impossible to obtain a target signal, so that the accuracy in the reproduction of the optical disk and in the tracking control and autofocus control of the objective lens deteriorates. In particular, in the light source unit which incorporates the light-guiding element and the light-receiving element for signal reproduction in a package, it is difficult to mount the light-guiding element and the light-receiving element for signal reproduction accurately at desired positions.
Insofar as the output of the forward laser light is indirectly feedback-controlled on the basis of the output of the backward laser light as in the optical pickup device having the above-described configuration, the actual output of the forward laser light varies, and it is naturally impossible to expect improvement in the accuracy of the return light from the optical disk which is incident upon the light-receiving element for signal reproduction.
Further, as for the optical disks such as a CD-R, a DVD-ROM, a DVD-R, an MD, and a PD, a laser light output during recording (an output which is about 10 times the laser light output during reproduction) is set for the specifications of each disk. For this reason, if the laser light output deviates from a light output range suitable for the optical disk subject to recording due to the variation of the output of the forward laser light in the above-described manner, a drawback occurs such as the distortion of the shape of the write pit. In addition, at the time of reproduction, if the laser light output becomes excessively large, there is a possibility that the data written in the optical disk may be deleted.
Here, it is conceivable to guide part of the forward laser light to the monitoring light-receiving element by using a reflecting plate or the like, but a new optical element such as the reflecting plate is required. If the new optical element is required, the cost of the light source unit becomes high, and it takes time in the operation of assembling the element.
With the conventional devices, the accuracy of the return light from the optical disk has been poor due to the above-described causes, so that it has not been possible to accurately effect the reproduction and recording of the optical disk and tracking and autofocus control of the focusing means. Accordingly, the present invention provides an optical pickup device which overcomes this problem.
According to an aspect of the present invention, there is provided an optical pickup device comprising:
a semiconductor laser for emitting forward and backward laser light from front and back emergent end faces thereof;
focusing means for focusing the forward laser light emergent from the front emergent end face of said semiconductor laser onto an optical recording medium;
a light-receiving element for signal reproduction for detecting return light from the optical recording medium, and
a light-guiding system for guiding the return light to said light-receiving element for signal reproduction, said optical pickup device being provided with a light source unit having an arrangement in which said semiconductor laser and said light-receiving element for signal reproduction being incorporated in a common package; and
shielding means, disposed inside said package of said light source unit, for preventing the light emitted from said semiconductor laser from being incident upon said light-receiving element for signal reproduction.
As described above, in accordance with the present invention, it becomes possible to improve the accuracy of the return light from the optical disk which is incident upon the light-receiving element for signal reproduction, and stabilize the tracking and focus controlling operation of the focusing means, thereby making it possible to accurately effect reproduction and recording of the optical disk and tracking and focusing control of the focusing means.